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‘Atmospheric river’ rain wallops Bay Area [Update]

A tremendous deluge is floating across the Pacific, bringing much-needed moisture but also risk of flooding

Dark clouds over a flat body of water. Photo by Sergey Ash/Shutterstock

[Update: A flood watch as been issued by the NSW, which will last until Saturday morning. Meanwhile rain has snarled traffic. In San Francisco, Sixth Street “under 6 to 8 inches of water,” according to SFGate. There is also reporting of minor flooding on the 101 in North Marin.

We will continue to update.]

The National Weather Service [NWS] warns that a tremendous storm in the form of an “atmospheric river” is heading straight for Northern California and the Bay Area, bringing much-needed rain but also some risk of flooding and other hazards.

In a notice posted Wednesday morning, NWS warns:

A potent atmospheric river will bring moderate to heavy rain to the Bay Area Thursday through Saturday. While confidence continues to increase for wet weather to return, a great deal of uncertainty remains on exact details of timing, rainfall amounts, locations of greatest impacts, and winds.

Rainfall amounts and subsequent impacts will be heavily dependent on the where the AR makes landfall. Stay tuned for additional forecast updates as the event approaches.

Since then it seems that the degree of certainty has increased, as the current regional forecast calls for a 100 percent chance of heavy rain in San Francisco Friday, with an 80 percent chance that the downpour will start pummeling the city by Thursday.

To a degree this is good news; the state is short of its usual winter rainfall and snowpack in spite of a spate of notable storms since late last year.

Map via NWS

But the oncoming downpour also comes with risks, as the forecast includes a note about the potential hazards of “localized flooding” (a risk to low-lying areas as the bay swells) and mudslides, which are a particular danger in the North Bay, where fire-scarred hillsides often cannot absorb heavy rain.

What exactly is an atmospheric river? A National Oceanic and Atmospheric Administration [NOAA] blog from 2015 explains:

Atmospheric rivers are relatively long, narrow regions in the atmosphere—like rivers in the sky—that transport most of the water vapor outside of the tropics. These columns of vapor move with the weather, carrying an amount of water vapor roughly equivalent to the average flow of water at the mouth of the Mississippi River. When the atmospheric rivers make landfall, they often release this water vapor in the form of rain or snow.

When an atmospheric river rises to push its way over coastal mountains and meets the cooler air higher in the atmosphere, this is particularly likely to spawn precipitation.

While atmospheric rivers can lead to dangerous conditions, NOAA notes that most of them are actually relatively mild affairs and provide critical precipitation for inland areas. But Bay Area residents in spots vulnerable to flooding or other storm hazards should still heed the warnings.